Why do the boys push a rock down from the top of the mountain lord od the flies​

To destroy Ralphs hiding spot in the thicket.

To kill Piggy

Explanation:

When Piggy and Ralph travel up to the savages camp to request his specs, Roger drops the boulder of the cliff onto Piggys head killing him instantly.

After SamnEric talk to Ralph he tells them he will be hiding in a certain bush. SamnEric betray him but the savages cannot get to Ralph there solution is to rolla boulder off the cliff into the thicket where he is hiding

the rhetoric device being used to persuade the audience is pathos.

you have to tell us what you're talking about or it's impossible to answer.